Capacitive structures of the type noted above are exemplified in U.S. Pat. Nos. 4,462,062, 4,448,340 and U.S. Pat. No. 4,531,268. These patents disclose that such a capacitive structure may be advantageously made by winding, in overlying relation on a drum, two webs of polymeric film, each providing a dielectric substrate. Each web of such film has a metallized coating on its upper surface, except for a narrow, longitudinal, demetallized zones, which may be scribed by laser means, and which divides the metallized coating into a relatively wide metallized area extending to and along one edge and a relatively narrow metallized strip extending to and along the other edge. The webs, which are of equal width, are offset laterally such that, as the webs are slit into parallel ribbons of uniform width before being wound on the drum, alternate ones of the successive layers of the overlying ribbons have their edges offset laterally in relation to the remaining layers. The resultant structure, which is called a "rope" because it has a tendency to be somewhat limp, is compressed at an elevated temperature so as to form a more rigid structure, which is called a "stick". A conductive, metallic mass, which typically is constituted by successive layers, e.g. an inner layer of aluminum applied by a metal-spraying process, a middle layer of copper applied by a metal-spraying process, and an outer layer of eutectic tin and lead solder applied by a dipping process, covers each edge of the overlying ribbon so as to provide electrical contacts to and between the metallized areas extending to and along such edge. The stick is sawed into discrete capacitors. In each capacitor, the conductive, metallic masses serve as electrodes, and a capacitive property is exhibited where the relatively wide metallized areas of the successive layers overlie one another. For further background, reference may be had to U.S. Pat. No. 3,670,378 and U.S. Pat. No. 4,229,865, which disclose other examples of capacitive structures of the type noted above.
Although capacitive structures of the type noted above as known heretofore have performed well in many applications, cracking can occur in their manufacture and in subsequent operations, particularly in capacitive structures more than approximately 0.1 inch thick. Cracking can introduce unwanted variability in the capacitive properties of such structures. Usually, when cracking occurs, a microscopic or macroscopic separation occurs between two layers of such a capacitive structure. Usually, such separation occurs between two central layers of the capacitive structure, at one of its sawed ends or at both of its sawed ends, whereby an unwanted cavity is formed in which ionic contaminants or other conductive contaminants can accumulate, which can reduce the dielectric resistance of the capacitive structure. Furthermore, some layers of the capacitive structure may break along a breaking line tending to be generally perpendicular to such separation, whereby the capacitance of the capacitive structure is reduced in an uncontrolled manner. Although there is no intention to be herein bound to any particular theory, it is believed that cracking occurs because opposite edges of the respective layers of metallized polymeric film are locked into the conductive, metallic masses providing electrical contacts, when the capacitive structures are subjected to thermal excursions, as explained below.
Typically, the discrete capacitors are heated to approximately 215.degree. C. for thermal normalization, then cooled. Cracking (when it occurs) is observed when the discrete capacitors cool. Typically, the inner layers of the metallic masses noted above are aluminum, which expands at a rate of approximately 25.times.10.sup.-6 cm/cm/.degree.C. Typically, the polymeric film is a polyester film, such as a poly(ethylene terephthalate) film, which expands at a rate of approximately 17.times.10.sup.-6 cm/cm/.degree.C. Aluminum has a thermal conductivity of approximately 2.37 watts/cm/.degree.C. as compared to poly(ethylene terephthalate) film, which has a thermal conductivity of approximately 1.54 .times. 10.sup.-3 watts/cm/.degree.C. Thus, each of the electrical contacts has a higher thermal conductivity and expands at a greater rate, as compared to the layers of metallized polymeric film. Consequently, the electrical contacts tend to separate the layers of metallized polymeric film before such layers expand. Furthermore, as cooling occurs, the electrical contacts tend to contract while the layers of metallized polymeric film tend to remain expanded. The outer layers of metallized polymeric film tend to cool before the inner layers cool, whereby the outer layers tend to form a rigid structure, to which the inner layers tend to conform as the inner layers cool. Since the upper, outer layers of metallized polymeric film, and the lower, outer layers of metallized polymeric film tend to compete for adherence of the inner layers of metallized polymeric film, the inner layers of metallized polymeric film thus tend to separate from each other, so as to form a crack.
A polyester film, such as a poly(ethylene terephthalate) film, is partially crystalline and tends to continue to crystallize with each temperature excursion. As such film continues to crystallize, such film tends to shrink. Such shrinkage tends to progress from the outer layers toward the inner layers and to contribute to cracking as discussed above.
Cracks seem to be also attributable to thermal expansion of adsorbed gases at metallized surfaces of the layers of metallized polymeric film. When the discrete capacitors are manufactured, and again when the discrete capacitors are soldered to substrates, the discrete capacitors can experience temperature changes from about 300K to about 500K, whereby such gases expand by a factor of 1.66 at constant pressure. While the expanding gases tend to separate the respective layers, the outer layers tend to cool before the inner layers cool and to form rigid, expanded structures, to which the inner layers tend to remain attached. Finally, as the central layers cool, a crack tends to form, which is bounded by concave surfaces of the layer of metallized polymeric film on opposite sides.
Because the respective layers tend to be well laminated, i.e., well adhered from layer to layer, very high forces are required to separate such layers. Sometimes, a crack steps through several layers before continuing between two adjacent layers. Such a step-form crack, which requires polymeric film layers to be sheared, demonstrates that such high forces tending to cause cracking are exerted. Frequently, a crack runs the entire width of a discrete capacitor and is sufficiently wide to allow light to pass through the crack, as may be easily seen under low magnification, possibly without any magnification. Cracks as wide as approximately 0.003 inch have been observed in discrete capacitors having a nominal thickness of approximately 0.16 inch. Occasionally, small cracks occur in the outer layers of such a structure, perhaps due to other crack-producing mechanisms.
Accordingly, there has been a need, to which this invention is addressed, for an improved capacitive structure, in which unwanted variability due to cracking is minimized.